Traditionally, when unacceptable background noise levels have been experienced on voice calls, the party experiencing the noise has simply turned down the volume setting, which reduces the background noise level but at the expense of the user's ability to hear the voice of the other party. Alternatively, in a manned conference-bridge type environment, a conference bridge operator can manually check the various lines of the conference call and turn down the volume on noisy lines.
In situations where the background noise is caused by a party being in a noisy location, solutions such as local mute and far-end mute are known. The obvious disadvantage to these approaches is that they do not distinguish between noise and voice.
Solutions such as highly directional handset microphones and speakerphones can do a good job of filtering out background noises, but they require the user to be positioned precisely or they, too, get filtered.
Prior to the development of electret microphones, telephone handsets used carbon microphones. Essentially, these are small canisters, filled with powdered carbon. The top of the canister was covered with a thin, highly flexible diaphragm. When sound waves pressed on the diaphragm, it would cause the carbon powder to be compressed, thereby reducing the electrical resistance of the canister. An interesting artifact of this design is that if sounds aren't loud enough to squeeze the carbon, they are not transmitted by the microphone. For this reason, carbon microphones are pretty good at filtering out the background noise at the user's location.
Electret microphones do not have this non-linear behavior. Because of their inherent ability to pick up low-amplitude sounds in addition to the user's voice, it became necessary to supplement the microphones with an expander circuit starting approximately 20 years ago. The expander circuit would measure the signal strength of the microphone and then, if the signal strength was below a predetermined threshold level, the transmitted signal would be attenuated electronically by an additional amount, perhaps 10 db.
When the background noise was at a level below the attenuator's threshold, the expander actually worked well. Needless to say, the expander was useless when the background noise was above the threshold, but the condition that was especially troubling was when the background noise was close to the threshold level, thereby causing the attenuator to kick in and out. For the listening party, the effect often sounded like heavy breathing.